1. Field of the Invention
The technical field of this invention is that of sealing bands for projectiles and especially for sub-caliber projectiles.
2. Description of the Related Art
Projectile bands have as an objective to provide during firing a seal to propellant gases between the wall of the barrel of the weapon and the projectile. These bands are usually placed in a surrounding groove of the projectile and, to provide such a seal, they are always slightly over-caliber (by approximately 10%).
A first problem encountered with known bands is to ensure the effectiveness of the seal during firing in worn barrels. Over-calibrating is not always sufficient to avoid leaks from surrounding gases, which reduce the projectile velocity, adversely affecting its efficiency and accelerating the wear of the barrel.
Attempts are in fact made to develop sub-caliber projectiles with ever increasing performance characteristics, i.e. with the greatest initial velocity possible (today this velocity is of the order of 1,800 m/s). Several types of bands have been developed for such sub-caliber projectiles.
According to established practice, sub-caliber projectiles consist of a sub-caliber (penetrator) core held by caliber sabot. The sabot is made of several segments (generally three) and releases the core upon exiting the weapon's barrel. The sabot usually has a sealing band positioned in a groove located on one section of the caliber sabot, a section usually called “pusher plate” since it is there that the propellant gases exert pressure.
European Patent EP307307 describes therefore a sealing band comprising a rear skirt attached to the cartridge casing, which provides during firing a low-pressure seal (pressure of the order of a few mega Pascal) and a front flange positioned in a groove of the projectile to ensure, like a classical sealing band, a high-pressure seal (pressure of the order of several hundreds of mega Pascal).
A low-pressure seal is essential in the first tens of milliseconds following ignition of the propellant charge contained in the combustible cartridge casing Indeed, the confinement provided by such a cartridge is inferior to that obtained with a metallic cartridge, and displacement of the projectile occurs at lower pressure. A seal that is not sufficiently sensitive to be operational from the time of ignition could have the risk of gases leaking through to the front of the projectile, which would decrease the efficiency of the propellant charge.
Such a device offers, however, some disadvantages.
The cartridge casing is fixed at the level of the skirt integral to the band, generally by riveting.
Rupture of the skirt during the passage of the weapon's forcing cone causes a decrease in the efficient width of the band. This results in a decrease in the firing ability of the projectile in worn barrels.
Furthermore, all efforts and mechanical constraints handled by the ammunition are transferred to the band, the sealing characteristics of which are at risk to deteriorate, which in turn could lead to dispersions from a ballistic point of view.
In order to further increase the velocity of projectiles, so-called “tractor” sabots have been developed, namely those wherein the pusher plate is located toward the front of the projectile.
The published document US-H-1353 describes such a projectile, which offers the advantage of a lightened sabot allowing for greater propellant powder charges in the cartridge.
However such a projectile should be fitted inside the barrel of the weapon, which compels the band to have a diameter inferior to the caliber to be fitted. Such an arrangement is detrimental to the sealing property due to a sub-caliber band. If on the contrary, an over-caliber band is used, it becomes difficult, or impossible to position the projectile due to induced friction.
Also, the guiding of such projectiles in the barrel is difficult to secure, and it is most often necessary to use ribs with a caliber extending to the back of the pusher plate. Such ribs hinder the sealing band assembly, which is most often fitted by hot deformation.